Image reproduction apparatuses such as VTRs and DVDs are adapted to receive a video signal compounded of a picture signal and a synchronization signal and reproduce the picture signal on a monitor screen.
Without any picture signal, the display screen shows a poor image, which looks like a picture of sandstorm. When the inputted video signal is deteriorated or weakened, noise components become relatively large, which again results in poor “sandstorm” pictures.
A conventional approach to circumvent such poor picture appearing on a display screen is to switch entire picture to a monochromatic blue back screen having a predetermined luminance when no picture signal is available.
In order to perform switching to the blue back screen (the switching hereinafter referred to as blue back processing) in the event that no video signal is available, a synchronization signal detection circuit for detecting the synchronization signal compounded in the inputted video signal is implemented to determine availability of the video signal. In accord with the detection result of the synchronization detection signal, the display screen is switched to a blue back screen (see for example, Japanese Patent Application Laid Open No. H5-227483, which will be hereinafter referred to as Patent Document 1).
The prior art as described in Patent Document 1 determines the existence or non-existence of a video signal using a synchronization signal detection circuit. However, if a video signal is inputted, the blue back screen is preferred to the video signal in the event that the inputted video signal is determined deteriorated. The criteria of determination as to whether the inputted signal is deteriorated or not depend on the type of the inputted video signal (e.g. compound signal or color difference signal) and on the type of the video tuner reproducing the inputted video signal. The prior art circuit disclosed in Patent Documents 1 has a drawback in that it cannot meet these criteria.
It is, therefore, an object of the present invention to provide a video signal judgment circuit for judging the condition of a video signal compounded of a picture signal and a synchronization signal, said circuit capable of judging existence or non-existence (availability) of a video signal and the degree of deterioration thereof, and regulating the criteria of judgment.